1. Field of the Invention
The present invention relates in general to monitors, and more particularly, to a method and apparatus for providing adaptive sharpness enhancement for a multi-frequency scanning monitor.
2. Description of the Related Art
Image quality depends on two major characteristics: contrast and sharpness. Contrast is the degree of difference between the light and dark regions. Sharpness is the degree of detail associated with the contours or edges of the image. While contrast tends to be a global feature, sharpness tends to be a local feature and is very much sensitive to how the pixels are displayed on the screen. Most commercial television sets provide adjustment of contrast and sharpness. The adjustment provides the user a means for varying the amount of contrast or sharpness according to his or her desire. Once the initial adjustment is made, no further adjustment is necessary. This is because television images are produced based on a fixed frequency as provided by the scanning circuitry. Adjustment of contrast and sharpness based on a fixed frequency system is thus simple.
Monitors that display digital images (e.g., graphics) typically use multi-frequency scanning to accommodate different image resolutions. The number of horizontal lines that can be displayed on the monitor screen is determined by how fast the horizontal scanning circuitry is driven. The higher the driving frequency, the higher the resolution. However, the change in scanning frequency usually causes degradation in image quality, and in particular, degradation in the sharpness of the image. When the system switches from one scanning frequency to another, the sharpness as set at one frequency does not produce the desirable sharpness quality at another frequency. The reason for this undesirable effect is that sharpness enhancement is typically performed at the edges or contours of the image, which is sensitive to scanning frequency.
In particular, sharpness enhancement involves accentuation at the edges or contours of the displayed image as shown in FIG. 1A. When the image goes from a light to a dark region, the edges are enhanced by increasing the image level at the edge and reducing the image level abruptly. This results in the display of an image with an effectively enhanced area EA1 (as shown in FIG. 2A) corresponding to the amount of abruptness, as indicated by the area A1 under the line a--a (as shown in FIG. 1A). Similarly, when the image goes from a dark to a light region, the edges are enhanced by decreasing the image level at the edge and increasing the image level abruptly. This results in the display of an image with an effectively enhanced area EA2 (as shown in FIG. 1B) corresponding to the amount of abruptness, as indicated by the area A2 above the line b--b (as shown in FIG. 1A). Thus, the amount of abruptness, as indicated by the area A1 or A2 under the line a--a or above the line b--b respectively, as shown in FIG. 1A, corresponds to the degree of sharpness. The effectively enhanced area EA1 or EA2 corresponding to the amount of abruptness is shown in FIG. 1B. The degree of abruptness is a function of three factors: (1) the difference L between the amplitude of the first and the second signal values (corresponding, for example, to the increased and decreased image levels), (2) the time interval T for the change from the first to the second signal values (or vice versa), and (3) the width of the signal (which, in FIG. 1A, is represented by T.sub.H).
In conventional systems, when a sharpness level is set at a particular frequency, the amplitude difference L as well as the time interval of enhancement T are both fixed. Accordingly, the enhanced area B1 or B2 corresponding to an input signal of a lower frequency (such as that shown in FIG. 2A), is substantially identical to the enhanced area A1 or A2 corresponding to an input signal of a higher frequency (such as that shown in FIG. 1A). However, since the interval (or width) T.sub.W of the input signal is an inverse function of frequency, the effective area EB1 or EB2 of the displayed image (corresponding to the input frequency with the lower frequency) is reduced (see FIG. 2B).
Accordingly, there is a need in the technology for a method and apparatus for providing adaptive sharpness enhancement for multi-frequency systems such as a multi-frequency scan monitor so that consistently and well enhanced images may be provided.